linux-kernel - [PATCH v5 1/2] x86: fix bitops.h warning with a moved cast

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Message-Id: <20200224225020.2212544-1-jesse.brandeburg@intel.com>
Date:   Mon, 24 Feb 2020 14:50:19 -0800
From:   Jesse Brandeburg <jesse.brandeburg@...el.com>
To:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de
Cc:     Jesse Brandeburg <jesse.brandeburg@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux@...musvillemoes.dk,
        andriy.shevchenko@...el.com, dan.j.williams@...el.com,
        peterz@...radead.org
Subject: [PATCH v5 1/2] x86: fix bitops.h warning with a moved cast

Fix many sparse warnings when building with C=1.

When the kernel is compiled with C=1, there are lots of messages like:
  arch/x86/include/asm/bitops.h:77:37: warning: cast truncates bits from constant value (ffffff7f becomes 7f)

CONST_MASK() is using a signed integer "1" to create the mask which
is later cast to (u8) when used, in order to yield an 8-bit value
for the assembly instructions to use. Simplify the expressions used to
clearly indicate they are working on 8-bit values only, which still
keeps sparse happy without an accidental promotion to a 32 bit integer.

The reason the warning was occurring is because certain bitmasks that
end with a mask next to a natural boundary like 7, 15, 23, 31, end up
with a mask like 0x7f, which then results in sign extension when doing
an invert (but I'm not a compiler expert). It was really only
"clear_bit" that was having problems, and it was only on bit checks next
to a byte boundary (top bit).

Verified with a test module (see next patch) and assembly inspection
that the patch doesn't introduce any change in generated code.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@...el.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@...el.com>
---
v5: changed code to use simple AND and XOR, updated commit message
v4: reverse argument order as suggested by David Laight, added reviewed-by
v3: Clean up the header file changes as per peterz.
v2: use correct CC: list
---
 arch/x86/include/asm/bitops.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index 062cdecb2f24..53f246e9df5a 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -54,7 +54,7 @@ arch_set_bit(long nr, volatile unsigned long *addr)
 	if (__builtin_constant_p(nr)) {
 		asm volatile(LOCK_PREFIX "orb %1,%0"
 			: CONST_MASK_ADDR(nr, addr)
-			: "iq" ((u8)CONST_MASK(nr))
+			: "iq" (CONST_MASK(nr) & 0xff)
 			: "memory");
 	} else {
 		asm volatile(LOCK_PREFIX __ASM_SIZE(bts) " %1,%0"
@@ -74,7 +74,7 @@ arch_clear_bit(long nr, volatile unsigned long *addr)
 	if (__builtin_constant_p(nr)) {
 		asm volatile(LOCK_PREFIX "andb %1,%0"
 			: CONST_MASK_ADDR(nr, addr)
-			: "iq" ((u8)~CONST_MASK(nr)));
+			: "iq" (CONST_MASK(nr) ^ 0xff));
 	} else {
 		asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0"
 			: : RLONG_ADDR(addr), "Ir" (nr) : "memory");

base-commit: ca7e1fd1026c5af6a533b4b5447e1d2f153e28f2
-- 
2.24.1